Monthly streaming services dont necessarily let you buy tracks to download onto an unconnected mp3 player (not connected online/wifi/etc)

You can download individual mp3s from amazon. Paying per track will be cheaper and less time consuming that ripping from youtube, and better quality AAnd once you buy them you own them, if kid loses mp3 player you dont need to start the whole process of ripping all over again.

Nobody really buys mp3s or cds anymore. I didn't even know it was still even possible to buy cds. "rip them"? People get their music through services like spotify or watch it online on youtube or other services. The days of "owning" mp3s are mostly a thing of the past. With that said, there are numerous free websites that will rip video or audio from youtube with a provided link. So, if your kid finds the music videos they like on youtube you could use these services (google is your friend) to rip the mp3 audio

The easiest (legal) way is to buy CDs, "rip" them, and encode the mp3s yourself. This can all be done with freely available software.

I got my kid an MP3 player for camp. Other devices aren't allowed. But where do I get music to download to add to it? I don't mind a subscription service to pay for for a while but I don't want to pay per song. Does this exist?